We are continuing to use the rat as a model to study the biosynthesis and metabolism of plasma apolipoprotein B (apoB), the structural protein of very low density lipoprpteins (VLDL). Evidence has been obtained that rat livers produce three forms of apoB, differing in their apparent molecular weights. Futhermore, the relative amount of the three forms produced appears to be regulated by diet and perhaps other physiological variables. This seems important because in other studies we have shown that all three forms of apoB are independently metabolized. Together, these findings point to a complex mechanism in the rat whereby the metabolic fate of hepatic VLDL and its lipid component is physiologically regulated by the form of apoB it contains. We have now developed a new and simpler method in vivo to measure the relative production rates of the individual forms of apoB. The method is based on the ability of Triton WR-1339, a nonionic detergent given I.V., to block the clearance of pulse-labeled apoB from the circulation. Experiments are proceeding with this new method. We have also begun to develop monoclonal antibodies to the various forms of apoB in the rat. Several promising mouse hybridomas have been obtained. We have also examined the protein profile of cholesterol-rich lipoprotein particles obtained from atherosclerotic aortal lesions of cholesterol-fed rabbits. Both cholesteryl ester-rich and inesterified cholesterol-rich particles exhibited the same protein pattern. The major aortic apoprotein is albumin, apparent MW= 66,000. Aortic lipoproteins also contain: Alpha-macroglobulin, apparent MW=185,000; transferrin, apparent MW=77,000; fibrinogen-Alpha, Beta, Gamma-chains, apparent MW=67,000, 56,000, 47,000; IgG-heavy and light chains apparent MW=55,000, 23,000. Interestingly, none of the serum apolipoproteins were observed on these cholesterol-rich aortic lipoprotein particles. Similar protein profiles were observed for particles isolated from human atherosclerotic lesions.